Typically audio systems, such as stereo systems, utilize a two-channel system. The two-channel system basically mixes multiple instruments, voices, effects and other audible signals into two signals or channels. Two-channel amplifiers drive loud speakers to reproduce these signals in audible form. Often a user wishes to power additional pairs of speakers from an amplifier.
Some of the newer systems do provide the option of additional zones so that a second pair of speakers can be used in locations such as a bedroom or office. The amplifiers are designed to not impose any additional load impedance.
The audiophile who wishes power a number of speakers may have to utilize a distribution amplifier and control system utilizing multiple amplifier channels. Such systems can both be technically complex and expensive.
Speakers may be wired in series or in parallel. Series wiring is relatively simple. When speakers are connected in this manner, the load impedance increases, as speakers are added the higher the impedance, as well as the higher impedance of the speaker reduces current draw from the amplifier. A common reason for raising the impedance is to lower acoustical output. Speaker output declines because the amplifier's power output decreases as load impedance increases. While it is possible to connect a number of speakers in series, it is suggested that the total equivalent load impedance for each channel be maintained between a safe level such as 16 ohms as most amplifiers are not designed to handle higher loads. Series wiring results in undesirable current hogging (uneven distribution of power) between series wired speakers.
In series installations, the amplifier sends an audio signal out through the positive speaker terminal into the first speaker. The signal is then sent from the first speaker to subsequent speakers until the circuit is completed. The negative terminal allows speakers to be connected to the amplifier's negative connection.
As mentioned above, in some cases a user wishes to power multiple speakers and will connect the speakers in parallel. However, with conventional parallel speaker wiring, the load impedance drops when speakers are wired in this fashion. The more speakers that are included in the system lower the impedance. The number of speakers that can be connected in parallel wiring is limited by the minimum load impedance that the amplifier is capable of driving and the power handling capacity of the speakers. In most installations, load impedance should be held to a minimum 4-8 ohms provided the amplifier can handle impedance that low.
By way of example, the load impedance for a parallel wired system can be calculated by the following equation:zt=(za*zb)/(za+zb)
where: zt=load impedance
za, zb represents the impedance of the speakers a, b
Using this equation, the impedance of each of the speakers are multiplied and the result is divided by the sum of the speaker's impedance.
Completing this equation, based on using two speakers having 8 ohms power rating it will be seen that the equation, when solved for zt, the net or equivalent load impedance for each channel is 4 ohms. Most standard amplifiers are rated in the 1 to 8 ohms range and therefore the minimum load impedance of most amplifiers is exceeded.